This invention relates to aqueous quenching media and processes using the same for quenching metal substrates.
Various methods of heat treating metal substrates are known and include heating a metal substrate to an elevated temperature and then cooling. The cooling step, which is known in the art as “quenching”, typically is performed rapidly and is accomplished by immersing a hot metal substrate in a liquid quenching medium, i.e. a quenching bath, which typically is water or oil.
When the quenching medium is water alone, very rapid cooling of the metal substrate occurs. Rapid cooling is not suitable for many types of steel, since it tends to produce excessive strain which distorts and/or cracks the metal, typically steel or aluminum. When the quenching medium is a hydrocarbon oil, a slower rate of cooling occurs. This can impart certain desirable physical properties to the metal substrate, including ductility in steel. Even though the slower cooling rate provided by oil quenching prevents or reduces excessive strain in the metal substrate, it often has the undesirable side-effect of preventing the metal substrate from adequately hardening.
Various aqueous media are available for quenching metal substrates and may include one or more of a polymer. One of the primary purposes of any quenchant is to mediate heat transfer and enhance the surface uniformity throughout the quenching process. For polymer quenchants, this is achieved by the formation and subsequent breakage and removal of an insulating film surrounding the hot metal part upon initial immersion.
Forming is a common problem encountered in quenching. Forming may be due to either chemical contamination or equipment design. Sources of chemical contamination include: detergents from cleaning solutions, metal working fluids, hydraulic fluids, i.e., process fluids. Equipment design problems include undersized reservoirs, return lines entering above the liquid level, air leaks and pump cavitation. Form formation around the metal substrate during quenching is undesirable because it enhances non-uniform cooling, resulting in increased distortion or cracking.
Another common issue in the quenching of metal substrates is microorganism proliferation. Particularly in aqueous metalworking fluids due to the water content, elevated temperatures, and contaminants, microbial contamination is common. Biocide products are often added to metalworking fluids, including aqueous quenchants, to protect against the uncontrolled growth of these organisms and prolong fluid life. They prevent corrosion and odors, extend fluid life and reduce down time caused by clogged or plugged lines or filters.
Bacterial actions alter fluid by destroying lubricants and corrosion inhibitors, and by generating corrosive organic acids, and salts. Because a cooling system cannot be stable or give consistent, predictable results with a significant bacteria load, controlling bacterial growth is critical for the long-term success of a coolant management program.
Commercial metal quenching compositions thus include a variety of components to handle the requirements of quenching, as well as to mediate the issues of forming and bacterial contamination. The addition of defoamers and antimicrobial or bioresistant components to aqueous quenching compositions adds to their expense and often to problems with environmental and health issues for workers in the quenching industries. There remains a current need for materials and methods to provide antifoam/defoaming properties and bioresistance to metalworking fluids, particularly fluids operating with agitation, such as quenchants.